Which day did World War I begin?

World War I officially began on July 28, 1914. This date marks the day when Austria-Hungary declared war on Serbia, after the assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist on June 28. The declaration of war set off a chain reaction of alliances and escalating tensions among European powers, leading to the outbreak of a widespread conflict that would involve nations from around the world. Hence, July 28 signifies the beginning of World War I, as it marked the first formal act of aggression among major nations in the war.